Indoor carbon dioxide (CO2) concentrations have been used for decades to evaluate indoor air quality (IAQ) and ventilation, and more recently in discussions of the risk of airborne infectious disease transmission. However, many applications of indoor CO2 measurement reflect a lack of understanding of the connection between indoor CO2 levels, ventilation and IAQ. In many cases, an indoor concentration of 1000 ppmv has been used as a metric of IAQ and ventilation without an understanding of its basis or significance. After many years trying to dissuade practitioners and researchers from using this or some other single concentration as an overall metric of ventilation and IAQ, an approach has been developed to determine a space-specific CO2 level that can be used as an indicator of the outdoor ventilation rate. The concept is to estimate the CO2 concentration that would be expected in a specific space given its intended or expected ventilation rate per person, the number of occupants and the rate at which they generate CO2. A calculation method is described for estimating the CO2 concentration in a given space at selected times after occupancy starts, which provides a more meaningful metric than a single value for all spaces. An online tool (QICO2, Quick Indoor CO2) has been developed to perform these calculations, and this report also contains a User Guide for that tool.